This invention relates to optical fibers and fabrication of preforms for use in drawing optical fibers.
Contemporary optical fibers are drawn from a cylindrical silica-glass object generally referred to as a preform. The preform has an axially symmetric structure that reflects the final structure of the optical fiber. The preform""s structure usually includes a central core, a middle cladding layer, and an outer overcladding or jacketing layer. To achieve the desired optical properties of the fiber, the core has a higher index of refraction than the cladding layer. Differences in indexes of refraction of the various fiber layers come from dopants, e.g., germanium and/or fluorine, which are incorporated during production of the preform.
As preform size continues to increase, in order to reduce fiber costs, the amount of overcladding relative to cladding also increases. The overcladding may comprise more than 85 percent of the fiber""s volume. The overcladding and its interface with the core-cladding largely determine mechanical properties such as draw breaking frequency and fiber limpness, i.e., curl. Though the overcladding determines mechanical properties, the core and cladding carry about 99 percent of the optical energy and primarily determine the fiber""s optical properties. The diminished impact of the overcladding on optical properties suggests fabricating the overcladding with processes that produce high mechanical quality but lower optical quality. Since such processes are often less costly, using them to fabricate the voluminous overcladding can substantially reduce overall production costs for preforms and for final optical fibers.
A sol-gel process is described in U.S. Pat. No. 5,240,488, (xe2x80x9c""488xe2x80x9d), which is incorporated by reference herein in its entirety. By the sol-gel process, overcladding tubes can be fabricated more cheaply than by processes using deposited soot as starting material. Fabrication of an overcladding tube using the sol-gel process involves casting a porous and opaque gel body from a colloidal sol of silica particles. The gel body is then dried, purified and sintered to produce the final silica-glass overcladding tube. A pre-made rod structure for the core and cladding is inserted into the overcladding tube, which is collapsed to produce the final preform.
In the sol-gel process, the treatment of the dried gel body has at least two stages. In a first stage, a purification treatment removes impurities, e.g., organic matter, water, and transition metals. These impurities are either present in the fumed silica starting material or in additives used to produce the gel body or are contaminants introduced during processing. In a second stage, a heat treatment sinters the gel body to close pores between silica particles and produce the final glass overcladding tube from the porous gel body.
A first embodiment features a process that produces a glass overcladding tube from a silica gel body. The process includes passing the gel body through a hot zone under conditions that cause partial sintering of the gel body and repassing the gel body through the hot zone under conditions that further sinter the gel body into a glass overcladding tube.
A second embodiment features another process for producing a glass overcladding tube from a silica gel body. The process includes subjecting one end of a cylindrical silica gel body to a hot zone until the end is at least partially sintered. The process also includes vertically passing the gel body through the hot zone to sinter the gel body. The act of passing causes the partially sintered end to enter the hot zone last.
Another embodiment features a manufacture for a preform. The preform has a core, a cladding layer, and an overcladding layer. The core, cladding layer, and overcladding layer each include silica-glass. The preform has an OD variation of 0.1 percent or less at one longitudinal position along the length of the perform.